High-temperature ammonium white mica from the Betic Cordillera

High-temperature ammonium white mica from the Betic Cordillera

American Mineralogist, Volume 93, pages 977–987, 2008
High-temperature ammonium white mica from the Betic Cordillera (Spain)
María Dolores Ruiz Cruz1,* and Carlos Sanz de Galdeano2
Departamento de Química Inorgánica, Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
2
Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Facultad de Ciencias, 18071 Granada, Spain
1
Abstract
High-temperature, ammonium-rich white mica has been identified for the first time in deep Paleozoic (and probably older) polymetamorphic schists from the Internal Zone of the Betic Cordillera
(Spain). Ammonium-rich white mica has been characterized by optical microscopy, X-ray diffraction,
infrared spectroscopy, elemental analysis, electron microprobe, and scanning and transmission electron
microscopy. High-temperature, ammonium-rich white mica shows some significant chemical differences with tobelite formed in hydrothermal and low-temperature metamorphic rocks. Although the
average formula, Ca0.09Na0.01K0.15(NH4)0.75(Al1.70Ti0.01Fe0.26Mg0.13)(Si2.99Al1.01)O10(OH)2, is typical of a
dioctahedral mica, the chemical plots reveal a clear deviation toward the trioctahedral field. Thus, the
increase in Fe + Mg contents is not accompanied by the parallel increase of Si contents, characteristic
of the phengitic substitution, which is characteristic of low-pressure conditions of formation. Chemical
differences are also accompanied by notable differences in the optical properties, both features suggesting that the term tobelite is not appropriate for this mica. Ammonium-rich white mica relics only
persist in some graphite-rich microdomains, defining the first schistosity. Textural relations indicate
that this mica formed during an older pre-Alpine metamorphic episode.
Keywords: Ammonium-rich white mica, Betic Cordillera, muscovite, SEM, TEM-AEM, tobelite
Introduction
Ammonium dioctahedral mica was discovered in hydrothermal deposits of Japan and termed tobelite by Higashi (1982).
From that time, the number of reports of tobelite and ammonium-bearing illite has progressively increased. Ammonium-rich
dioctahedral mica appears in two main geologic environments:
(1) hydrothermal deposits and (2) very low-grade metapelites. In
the second case, tobelite can have a wider distribution, especially
in sedimentary terrains rich in organic components, such as many
Carboniferous formations (Juster et al. 1987; Daniels and Altaner
1990; Sucha et al. 1994; Liu et al. 1996; Nieto 2002).
Lack of tobelite in higher-grade rocks has been attributed
to devolatilization processes occurring between 300 and 500
°C (Juster et al. 1987). Nevertheless, the available DTA curves
(e.g., Higashi 1982) indicate that loss of NH4 begins at approximately 550 °C, and tobelite must be, theoretically, stable
in metamorphic rocks below this temperature. Indeed, tobelite
and its analogous trioctahedral mica have been synthesized at
relatively high temperatures (up to 550 °C) (Eugster and Munoz
1966; Boss et al. 1988; Sucha et al. 1998; Harlov et al. 2001a,
2001b). Surprisingly, high-temperature, ammonium-rich white
mica had not been previously described, this in spite the fact
that high NH4 contents (up to 3000 ppm) have been measured in
micas of various metamorphic grades (Honna and Itihara 1981;
Itihara and Suwa 1985; Duit et al. 1986; Visser 1992; Boyd and
* E-mail: [email protected]
0003-004X/08/0007–977$05.00/DOI: 10.2138/am.2008.2760
977
Philippot 1998; Mingram and Braüer 2001).
We present the first finding of high-temperature ammonium
dioctahedral mica, which has been identified in polymetamorphic
schists from the Internal Zone of the Betic Cordillera (Alpujárride Complex). Nevertheless, ammonium-rich white mica is very
scarce in these rocks, which is a possible explanation of why it
has not been previously reported.
Geologic setting
The Betic Cordillera, the westernmost European Alpine chain,
has been traditionally divided into a northern external domain
including the Prebetic and the Subbetic zones; an intermediate
domain, the Flysch units from the Gulf of Cádiz area; and a
southern domain, the Internal Zone. The Alpine orogeny (Upper Cretaceous to Miocene) involved collision of the Internal
and External Zones. The collision was accompanied by intense
structural deformation and metamorphism, which mainly affected
the Internal Zone. Structurally, the Internal Zone is formed of
four tectonically superimposed complexes (Egeler and Simons
1969; Puga et al. 2002), from bottom upward, the Veleta and the
Mulhacén (both generally included in the Nevado-Filábride), the
Alpujárride and the Maláguide (Fig. 1). The geotectonic relationships between the Maláguide, the Alpujárride, and the NevadoFilábride are essentially constant in the Betic Cordillera. The
Maláguide Complex tectonically overlies the Alpujárride Complex, and this later overlies the Nevado-Filábride Complex.
The Alpujárride Complex shows sequences comprising
poorly dated Palaeozoic (and probably older) terrains (Egeler